It is important in the design of various types of equipment to limit the transmission of noise and vibration to adjacent structures. In particular, in an aircraft such as a helicopter, it is required that cabin noise be limited so as not to disturb the cabin occupants. A major source for noise in such an aircraft is caused by high frequency vibration transmitted to the cabin area from the overhead drive structure. A major cause of the high frequency vibration is the meshing of gear teeth in the rotor transmission, and the gear clash of the bevel pinions with the main bull gear. Typically, the transmission is mounted directly to an airframe supporting structure, which also comprises the frame for the cabin. The high frequency vibrations, along with other such vibrations hereinafter referred to collectively as "noise" are transmitted through the structural members which mount the transmission to the airframe. It is estimated that 90% of the cabin noise is structurally transmitted, rather than airborne transmitted.
As power requirements have increased, the amount of noise transmitted through the structure to the cabin has correspondingly increased. At the same time, noise requirements have become more restrictive, and it has been found that at these increased power levels, the noise level within the cabin, particularly during high power maneuvering, its too high.
Various attempts have been made to reduce noise, such as incorporating elastomeric sleeve members having high spring stiffness about the bolts which are used to mount the transmission to the airframe. However, the largest noise component is made by lateral vibratory loads and these bypass the sleeve members. Additionally, such sleeves allow the bolts holding the transmission to the airframe to lose torque over time as the elastomer creeps or flows, which is an undesirable result.
Other attempts have been made to reduce noise by adding cabin insulation or complex noise suppression systems, such as those issuing counter frequency noise to eliminate the noise generated, but these attempts add weight and complexity to the aircraft, and are also fairly costly. Consequently, they are not considered satisfactory solutions to the problem.